Land vehicles including cars, trucks, trains, and mass transit systems use disc brakes to slow and/or stop the vehicle. Disc brake systems generally include a rotor and caliper. The rotor is mounted to turn with the wheel of the vehicle. The caliper includes brake pads that are forced into frictional contact with the rotor to slow and or stop rotation of the wheel. Conventional cast iron brake rotors are relatively heavy. These rotors wear during braking, generating dust. Alternatives to conventional cast iron brake rotors can reduce weight as well as contribute to better fuel economy, reduced both air and water pollution, and enhanced vehicle acceleration. Reduced weight materials to cast iron include aluminum and titanium; however, their surface tribology in a friction application, in contrast to cast iron, lacks the necessary performance to function as a brake rotor. It has been known to add ceramic particulate to a metal matrix to increase friction for improved stopping power and to enhance wear resistance, which also has the advantage of producing little to no dust in a friction application.
Many conventional processes have drawbacks when applied to forming brake rotors with desirable features. For example, some rotors formed of aluminum with a ceramic coating applied by a conventional plasma spray technique have unsatisfactory residual porosity as well as insufficient adhesion of the coating to the rotor substrate for vehicular rotor applications. Brake rotors mostly free of porosity are enabling for reliability and good heat transfer.
Conventional stir casting used to produce composites and particularly aluminum-ceramic particle composites inherently results in unacceptable porosity for most vehicular rotor applications. In addition, ceramic particle loading of at least 30 to 50% by volume is desirable for high wear resistance and braking performance of vehicles with relatively high momentum. Conventional stir casting is not known to produce pore free composites at this necessarily high particle loading. In conventional stir casting, ceramic particles are incorporated throughout a molten aluminum alloy. As the insoluble particulate is stirred into the liquid aluminum, the viscosity of the mixture increases with the volume of particulate added. The increase in viscosity is also related to the size of the particulate. Generally, the smaller the particulate, the greater the increase in viscosity. However, fine particle sizes are used, preferably below approximately 50 μm for good braking performance. A conventional stir process to incorporate fine ceramic particles into the aluminum matrix for brake rotor applications is limited to approximately 25% by volume, depending on the actual particle size, because stirring and casting of higher volume loading and viscosity mixtures become impractical.